The invention relates to a method and apparatus for controlling an induction motor, and particularly to a method and apparatus for providing a constant control of air flow, fluid flow, fluid pressure or other physical output of an induction motor in such a way as to increase the efficiency of the motor.
It is commonly known in the art to provide an air handling system such as a heating, ventilating or air conditioning ("HVAC") system with a blower or fluid pump that either pushes air over or draws air across a heat exchanger or cooling coil to heat or cool the air, respectively, and transfer the air through a system of ducts and vents to a room or rooms where a thermostat is located. The thermostat provides feedback to the system to indicate the temperature in the room or rooms. In this way, the temperature of the air in those rooms is controlled. The blower typically includes a motor and the HVAC system also usually includes a controller for controlling the motor in response to various parameters such as room air temperature, air flow rate, motor speed, and motor torque.
It is also known that the efficiency of the heat transfer between the air and the heat exchanger cooling coil is directly dependent upon the flow rate of air across the heat exchanger or the cooling coil. Moreover, it is known that the total system efficiency can be maximized by maintaining the flow rate of the air at a specific set point. The set point or flow rate at which the system is most efficient is often determined empirically (typically by the manufacturer of the HVAC system), and is programmed into the controller of the motor. As vents in the system are opened or closed, however, the load on the motor changes, thereby changing the motor speed, blower output and stator current. The changing loads experienced by the motor make precise control of the blower output extremely difficult. One method and apparatus for controlling a blower motor under such conditions is shown and described in U.S. Pat. No. 5,656,912, which is incorporated herein by reference.